Contrast is comparing or juxtaposing two things which are very different. An example in visual art or poetry is the contrast of light and dark.
The general dramatic contrast in this ballad is the contrast between images of life and death. The image of death is symbolized by the absence of Bonnie George Campbell. "Bonnie" means physically attractive or excellent. That image of death is contrasted by the images of life symbolized by George’s mother, his bride, the lush meadow and finally, his horse.
Lines 13-16 represent the contrast between the life of nature and death. The grass of the meadow is alive and the corn has not even been shorn (harvested or shuck). The world George’s horse comes back to is teeming with life. This is in dramatic contrast to the death of George. George’s absence and the bloody saddle convey that George has died in battle.
“The meadow lies green,
the corn is unshorn,
But bonnie George Campbell
will never return.” (13-16)
This is a ballad but it also could be called a lament.
This may be a reference to a fallen soldier, last name of Campbell, at the battle of Glenlivet in October 1594.